A configuration in a wireless communication system, particularly, in a mobile communication system, in which a plurality of antenna units equipped with part of functions of a base station are physically stretched out and are used as remote units is being studied. Here, an antenna unit refers to a unit that is equipped with a transmission interface, a wireless transceiver, and an antenna. A base station with a plurality of remote units stretched out therefrom functions as a central unit to control the remote units. In the communication scheme for performing communication between the central unit and the remote units, two system configurations which differ from each other in terms of distribution of functions between the central unit and the remote units are being studied.
One of them is referred to as full centralization. As shown in FIG. 1, in this system configuration, a central unit 10 is equipped with functions of a physical layer excluding antenna units and functions of a data link layer and higher layers, and remote units 20 each equipped only with an antenna unit are stretched out. The other is referred to as partial centralization. As shown in FIG. 2, in this system configuration, a central unit 10 is equipped with functions of a data link layer and higher layers, and remote units 20 each equipped with functions of a physical layer including an antenna unit are stretched out. Moreover, as shown in FIG. 3, a configuration in which part of the functions of the physical layer is left in the central unit 10 in the configuration of the partial centralization is also being studied.
Today, the more widely used system configuration is full centralization. The communication scheme between the central unit 10 and the remote units 20 in this configuration uses a digital radio over fiber (RoF) technique as typified by a common public radio interface (CPRI).
On the other hand, in a mobile communication system, an area covered by a single base station is referred to as cell, and there is a problem of a phenomenon in which when a mobile station reaches the edge region of a cell, wireless signals being transmitted from a desired base station interfere with wireless signals being transmitted from an adjacent base station, resulting in a significant reduction in transmission rate between the base station and the mobile station. As a means for solving the problem of such interference between the signals from the cells, as shown in FIG. 4 and FIG. 5, a coordinated multi-point transmission/reception (CoMP) technique in which adjacent base stations (FIG. 4), a central unit 10 and a remote unit 20 (FIG. 5), or remote units 20 (FIG. 5) operate in cooperation with each other to communicate with a terminal 30 (mobile station) which is positioned at the edge of a cell 92 is being studied. It is to be noted that in FIG. 4 and FIG. 5, reference symbol 91 denotes a core network. Moreover, as one of the techniques for implementing the CoMP in uplink, there is a technique known as joint reception (JR), in which signals are received at different base stations to improve signal quality, and in this case, a plurality of base stations that receive signals in cooperation with each other may be seen as reception antennas for multiple-input multiple-output (MIMO) transmission. At this time, MIMO transmission when the number of terminals that transmit signals is one is referred to as single user (SU)-MIMO, and MIMO transmission when a plurality of terminals transmit signals simultaneously is referred to as multi user (MU)-MIMO (for example, refer to Non-Patent Document 1).
Examples of techniques for signal detection in MIMO transmission include: minimum mean square error (MMSE), in which signals are detected by means of a matrix operation with a reception weight matrix generated based on channel information estimated from reception signals; successive interference cancellation (SIC), in which signals are detected sequentially from higher quality signals; and maximum likelihood detection (MLD), which is also referred to as a maximum likelihood decision method, in which all combinations of transmission signals are compared with reception signals to perform discrimination (for example, refer to Non-Patent Document 2). In MLD, there are a receiving process that uses hard decision, and a receiving process that uses soft decision. In the hard decision, code words corresponding to estimated transmission signals are output as an output of the MLD, whereas in the soft determination, likelihood information of each bit of estimated transmission signals is output (for example, refer to Non-Patent Documents 3 and 4). Moreover, in the MLD process, the number of transmission signal vector candidates increases exponentially in accordance with the number of modulation levels and the number of transmission antennas. Therefore, techniques of reducing the amount of computation in the MLD process at each remote unit are being studied (for example, refer to Non-Patent Document 3).
FIG. 6 shows a signal transmission flow in the case of performing CoMP by means of JR on a full centralization system configuration. In FIG. 6, the number of remote units that operate in cooperation with each other is two, and a single antenna 21 is provided in each of these remote units 20. However, the number of remote units that operate in cooperation with each other need not be limited to two, and the number of antennas to be provided in each remote unit 20 may be more than one. Furthermore, in FIG. 6, the number of terminals that transmit signals is one, and the number of antennas of a terminal 30 is two. However, the number of terminals 30 that transmit signals may be more than one, and the number of antennas of the terminal need not be limited to two. Transmission signals s1 and s2 are transmitted from two antennas 31 of the terminal 30, and are received as reception signals r1 and r2 by the antennas 21 of a remote unit #1 and a remote unit #2, via a wireless transmission path expressed by a channel matrix H of elements hij (i=1, 2, and j=1, 2). At this time, noises n1 and n2 are added to the reception signals during a reception process of RF signals in radio frequency (RF) receiving units 22. The relationship between these transmission signals, channel matrix, reception signals, and noises is expressed as Equation (1) below using vectors and matrices.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                            r        =                                            Hs              +              n                        ⇔                          [                                                                                          r                      1                                                                                                                                  r                      2                                                                                  ]                                =                                                    [                                                                                                    h                        11                                                                                                            h                        12                                                                                                                                                h                        21                                                                                                            h                        22                                                                                            ]                            ⁡                              [                                                                                                    s                        1                                                                                                                                                s                        2                                                                                            ]                                      +                          [                                                                                          n                      1                                                                                                                                  n                      2                                                                                  ]                                                          (        1        )            
The reception signals received by the remote units 20 undergo signal conversion such as conversion into CPRI signals, in signal conversion units 23, and they are then transmitted to the central unit 10. In the central unit 10, signal conversion units 11 perform signal conversion on the signals that have been transmitted, and a MIMO signal detection unit 13 performs a MIMO signal detection process based on the reception signals r1 and r2 received at the remote units 20. As a MIMO signal detection process, MLD is used, for example. When the transmission signals s1 and s2 have both been modulated by means of binary phase-shift keying (BPSK), in the MIMO signal detection by means of MLD, four transmission signal vector candidates sc1, sc2, sc3, and, sc4 expressed as Equation (2) to Equation (5) below are multiplied by a channel matrix He, which has been estimated by means of channel estimation in a channel estimation unit 12 from the reception signals r1 and r2, to generate reception replicas rc1, rc2, rc3, and re4 expressed as Equation (6) to Equation (9) below.
                    [                  Equation          ⁢                                          ⁢          2                ]                                                                      s                      c            ⁢                                                  ⁢            1                          =                  [                                                                      e                                      j                    ⁢                                                                                  ⁢                    0                                                                                                                        e                                      j                    ⁢                                                                                  ⁢                    0                                                                                ]                                    (        2        )                                [                  Equation          ⁢                                          ⁢          3                ]                                                                      s                      c            ⁢                                                  ⁢            2                          =                  [                                                                      e                                      j                    ⁢                                                                                  ⁢                    0                                                                                                                        e                                      j                    ⁢                                                                                  ⁢                    π                                                                                ]                                    (        3        )                                [                  Equation          ⁢                                          ⁢          4                ]                                                                      s                      c            ⁢                                                  ⁢            3                          =                  [                                                                      e                                      j                    ⁢                                                                                  ⁢                    π                                                                                                                        e                                      j                    ⁢                                                                                  ⁢                    0                                                                                ]                                    (        4        )                                [                  Equation          ⁢                                          ⁢          5                ]                                                                      s                      c            ⁢                                                  ⁢            4                          =                  [                                                                      e                                      j                    ⁢                                                                                  ⁢                    π                                                                                                                        e                                      j                    ⁢                                                                                  ⁢                    π                                                                                ]                                    (        5        )            [Equation 6]rc1=Hesc1  (6)[Equation 7]rc2=Hesc2  (7)[Equation 8]rc3=Hesc3  (8)[Equation 9]rc4Hesc4  (9)
Squared Euclidean distances between the reception replicas and the reception signal r expressed as Equation (1) are calculated as Equation (10) to Equation (13) below, and the transmission signal vector candidate that corresponds to the reception replica with the minimum squared Euclidean distance is determined as an estimated transmission signal.[Equation 10]∥r−rc1∥2  (10)[Equation 11]∥r−rc2∥2  (11)[Equation 12]∥r−rc3∥2  (12)[Equation 13]∥r−rc4∥2  (13)
Finally, the determined transmission signals, after having been output as corresponding code words c1 and c2, undergo a decoding process in decoding units 14, and they are supplied to a media access control (MAC) function unit 15 of the data link layer as bit sequences b1 and b2. Here, after the signal conversion in the central unit 10, a process for receiving multi-carrier signals, such as those in orthogonal frequency division multiplexing (OFDM), may be performed on the reception signals. Moreover, transmission signal vector candidates do not always have to be limited to mapping on a complex plane as shown in Equation (2) to Equation (5), and another mapping may be performed. Furthermore, an MLD process such as the one described above is a receiving process that uses hard decision. However, a configuration which uses a receiving process that uses soft decision to output likelihood information may be employed. In this case, the output likelihood information is input to a soft input decoder such as a turbo decoder, undergoes a decoding process, and it is then output as a bit sequence.
However, in signal transmission in such a full centralization system configuration, signals obtained by performing sampling and quantization on the reception signals r1 and r2 are transmitted. Therefore, there is a problem in that the transmission capacity between the remote units and the central unit becomes extremely large. For example, when CPRI is used for signal transmission, if communications in a wireless section are performed at a transmission rate of 75 M bit per seconds (bps), the required transmission rate between the remote units and the central unit is 1,228 Mbps, which is approximately 16 times 75 Mbps.
On the other hand, FIG. 7 shows signal transmission when CoMP using JR is performed in a partial centralization system configuration, which, in comparison with a full centralization system configuration, can reduce the transmission capacity required between the central unit 10 and the remote units 20. As with FIG. 6, in FIG. 7, the number of remote units that operate in cooperation with each other need not be limited to two, and the number of antennas to be provided in a single remote unit 20 may be more than one. Moreover, the number of terminals 30 that transmit signals may be more than one, and the number of antennas of each terminal 30 need not be limited to two.
The decoding process, which is to be performed in the central unit 10 in full centralization, is performed in the decoding unit 26 of each remote unit. In partial centralization, in contrast with full centralization, data transmitted between the central unit 10 and the remote units 20 are bit sequences b1 and b2, which have undergone decoding. The data transmitted between the central unit 10 and the remote units 20 is not data of signals obtained by performing sampling and quantization on wireless signals, but is bit sequence data after decoding. Therefore, the transmission capacity between the central unit 10 and the remote units 20 becomes significantly small, compared with the case of full centralization. However, since the functions of the physical layer are distributed in partial centralization, there is a problem in that MIMO signal detection that uses reception signals r1 and r2 both as shown in FIG. 7, that is, CoMP, cannot be performed in the MIMO signal detection unit 25. Moreover, in the case of the configuration of FIG. 7, channel information that can be estimated by the channel estimation unit 24 of each remote unit 20 is he11 and he12 only at the remote unit #1, and it is he21 and he22 only at the remote unit #2. It is to be noted that after RF reception has been performed at a remote unit, a process for performing reception of multi-carrier signals such as those in OFDM on reception signals may be performed.